This invention relates generally to a screw thread coupling and in particular to an improvement of the screw thread coupling disclosed in U.S. Pat. No. 4,799,844.
In the art of percussion drilling, such as for mining, quarrying water well drilling, and the like, various components of a drilling tool are frequently coupled together by threaded screw structures. In these coupled drilling components, the thread design is of considerable importance since failure often occurs in the screw structure. Whenever failure in the screw structure does occur, the crack usually initiates at the thread root. This is due to the high stress concentration located at the root of the thread when the screw structure is subject to severe loading.
Traditionally, the root portion has been configured as a portion of a circle, tangentially adjoining the two flanks of successive thread turns. Generally speaking, the stress concentration along the thread root is an inverse function of the radius of that circle, i.e., the larger the radius of the circle, the lower the stress concentration. However, when the size of the radius surpasses a maximum allowable value, the stress concentrations at the ends of the root, where the flanks of the adjacent thread turns tangentially adjoin the root, become very high and thus provide starting points for crack propagation. Alteratively, when the size of the radius defining the circular root curvature decreases below a minimum allowable value, the stress concentration becomes very high at the bottom of the root. Thus, circular root curvatures are confined to radii sizes falling between certain maximum and minimum limits, depending on such factors as thread pitch and the like.
An improved screw structure has been disclosed in U.S. Pat. No. 4,799,844 wherein the screw structure of each of the threadedly interconnected drilling components includes a root portion configured as a portion of an ellipse. That elliptical root curvature provides a larger equivalent radius at the thread root which results in a decreased stress concentration along the root. Notwithstanding the benefits attainable by that screw structure, room for improvement remains.
In that regard, it may occur that the screw structure is utilized to interconnect two components having different expected life spans, such as certain elements of a percussion drill, for example. Depicted in FIG. 1 is a down hole percussion drill which includes a drill bit 10 mounted at the end of a drill string. The drill string includes a driver sub 12 which slides over a rear end of the drill bit and which is connected to the drill bit 10 by a spline connection, i.e., by eternal splines 14 on the drill bit and internal splines (not shown) on the driver sub 12. The drill bit 10 is thus constrained to rotate with the driver sub 12 but is capable of limited axial movement relative thereto. The driver sub 12 includes an external or male screw thread 16 which is threadedly connected to an internal or female screw thread 18 of a cylindrical case 20. Slidably disposed within the case 20 is a piston 22 which is axially reciprocated by a conventional air-actuated mechanism in order to apply percussive forces to the rear end of the driver sub. Those forces are transmitted through the driver sub to the drill bit to enhance the penetration rate of the drill bit. An upper end of the case 20 is threadedly connected to a top sub (not shown), and the latter is threadedly connected to another element of the drill string, and so on.
The use in such a percussion drill of a screw structure having an elliptical root as disclosed in U.S. Pat. No. 4,799,844 has reduced the occurrence of stress fractures at the thread roots. However, other factors come into play during a drilling operation to wear out and shorten the life span of the drilling elements. That is, the percussive loads transmitted through the drilling components produce wearing of the thread flanks; contact with the surrounding earth and rock formations produces an erosion of the outer surfaces of the drilling elements. Also, the drive splines of the driver sub are susceptible to wear as the result of impacting and rubbing against the splines of the drill bit. In practice, it has been found that those factors affect some of the drilling components more so than others, whereby the various components tend to wear out at different rates and require replacement at different times. For example, the cylindrical case 20 tends to wear out less rapidly than the driver sub 12. Furthermore, the present inventor has observed that the case often tends to require replacement as the result of thread wear, whereas a driver sub normally must be replaced well before its thread has worn out.
It will be appreciated that the replacement of a drilling component is quite expensive in that it requires that the drilling operation be stopped so that the drill string can be raised. An enhancement of productivity and efficiency could thus be realized if the frequency of such work stoppages could be reduced.